Short answer
AC reverses direction periodically. DC flows in one direction. US household outlets provide AC, while a battery supplies DC at its terminals. Electronic devices may convert one to the other internally. 1 2
Both transfer electrical energy; the central difference is whether charge-flow direction periodically reverses.
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At a glance
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| Question | Alternating current (AC) | Direct current (DC) |
|---|---|---|
| Flow direction | Periodically reverses | One direction |
| Familiar source | Household mains | Battery |
| Can voltage vary? | Yes | Yes, even for DC |
The table summarizes the cited definitions and the article’s stated scope. 1 2
What each thing is
Alternating current (AC). Electric current whose direction reverses periodically. 1
Direct current (DC). Electric current that flows in one direction, even if its strength is not perfectly constant. 2
Key differences
OpenStax defines the distinction by direction of charge flow. A perfectly steady magnitude is common in simple DC examples but is not the essential requirement; a one-direction pulsing current can still be DC. 1 2
How to tell them apart
Look for AC or DC on equipment labels and power supplies. A waveform crossing through zero and reversing polarity periodically is alternating; unidirectional flow is direct. Do not probe live power without suitable equipment. 1 2
Where they overlap
Both involve moving charge and can power the same device through conversion. Modern power systems use AC transmission and many DC components. 1 2
Edge cases
Rectified AC may be pulsing DC until filtered. Some waveforms include both AC and DC components. The historical “war of currents” does not imply one type is universally better for every application. 1 2
Why the distinction exists
Different current forms suit different generation, transformation, storage and device requirements; historically AC systems facilitated long-distance transmission. 1 2
Common misconceptions
DC does not have to be perfectly constant in magnitude. AC does not mean power travels back and forth with no useful energy delivered. 1 2
Examples
A wall outlet supplies alternating current. A flashlight battery supplies direct current to its circuit. 1 2
Sources
Sources checked October 3, 2026.
- OpenStax — Alternating Current versus Direct Current. AC and DC definitions.
- U.S. Energy Information Administration — Electricity Explained. Electricity history.